Beginning in 1930, we're dropped into St. Luke's Home For Boys, where a young Calvin Evans is being chastised by a nun for being curious. He gets to class late, earning himself a demerit, and then opens his Bible Studies book, showing he's hidden a chemistry leaflet inside.
As Calvin grows, his understanding of chemistry seems to grow more respected at the boys' home, and he's shown advising a flourishing candle-making operation to run smoothly so the home can continue to raise money for the boys.
One day, an older man arrives at St. Luke's, and all the boys flurry in excitement. Calvin gets called out of class by one of the Fathers in charge at the home, seeming to believe he'll be adopted out, but the Father only relieves him of his chores for the day. When Calvin asks about the man, the Father shares that he's too indispensable to St. Luke's
In 1946, Calvin is at Hastings Lab, having his headshot taken. He criticizes one of the experiments happening in the background at the lab, then says he needs his lab without distractions. The head of the lab, an older man, tells him it'll be done. Calvin heads to Harvard for a presentation, and one of the students he's spoken to is revealed to be Reverend Wakely, whose first name is Curtis. Curtis writes Calvin a letter later that night, sharing that he enjoyed the presentation but wonders if Calvin may have overlooked the presence of God.
The pair write each other letters, Calvin welcoming the questioning and inviting Curtis to share more thoughts. As Calvin and Curtis correspond, they open up to one another, and Calvin begins sharing literature, like Darwin, that Curtis may enjoy.
At his new home, Calvin plays music and dances around his living room when Harriet and Charles knock on the door. Charles is hesitant, considering Calvin is white, but the pair head into his home and all chat. When Calvin learns that Harriet is in legal aid, he shares that he's been receiving letters from people who are asking for money, pretending to be his parents, and threatening him, asking if she can help. Harriet agrees, and a friendship has been forged between the trio.
Though Calvin is feeling lonely, he continues researching and corresponding with Curtis, who begins asking more personal questions about Calvin's life. On a date, Calvin is a bit uncomfortable with women, as he can't connect. In a letter to Curtis, he shares he feels he may be better off alone. In a crisis of faith, Curtis shares what he's going through with Calvin. Curtis' Father has gallbladder cancer and is rejecting treatment, favoring prayer. Even in a moment of hurt, Curtis takes the time to recognize that something divine has brought Calvin into his life.
In 1951, Calvin wrote Curtis a letter about Elizabeth after meeting her and beginning to fall for her. In the letter, he explains that he wants to give Elizabeth everything, that she is his how and why. Curtis is Excited about Calvin and shares that he has great news of his own - his Father has agreed to treatment.
Around Christmas, Calvin goes into a jewelry store, shopping for Elizabeth without fully intending to. He describes her to the salesgirl, explaining she's simple in the best way possible. The salesgirl finds a ring Calvin loves, and he purchases it. At Hastings, Calvin nearly proposes several times but is far too nervous. Elizabeth moves in with Calvin, who asks Curtis for his advice on proposing. Curtis explains he shouldn't overthink it, but Calvin continues to struggle.
After receiving more correspondence from people looking to scam him, Calvin snaps at Elizabeth in their lab. He apologizes to Elizabeth for losing his temper, who softly asks what's happening once they're at home. The couple debriefs on Calvin's life. He shares that his parents abandoned him before they died, so people pretending to be his family make things hurt him more. As Calvin and Elizabeth's relationship deepens, his correspondence with Curtis lessens. He writes a letter the day of his accident, putting it in the mailbox before taking Six-Thirty on the fateful run that ends his life. Curtis, in response, writes that he and his family are set to return to California and hope to meet Calvin in person.
After Calvin's death, Hastings returns Curtis's letter to the sender, leaving him confused.
In 1958, Curtis arrives at Elizabeth's home with all his letters from Calvin. Mad takes the letters, organizing them as Curtis and Elizabeth discuss their connections with Calvin. Curtis says he likely connected with Calvin because they both knew how little they knew about the world, while Elizabeth shares that she and Calvin found faith in each other.
At St. Luke's Boys Home, Mad and Elizabeth arrive for a visit. They meet with the Father in charge of the home, the same man we saw earlier with Calvin. He lies to Elizabeth and Mad, explaining he has no files from the years that Calvin would've been there. Mad, who doesn't believe the Father, heads to the library and looks through books to see if she can find something Calvin checked out. She discovers a copy of Great Expectations, which Calvin had signed out in 1929 and was initially donated to St. Luke's by the Remsen Foundation.
Elizabeth, understanding the connection, realizes they have a new lead.